Populaatioinversio

Populaatioinversio, or population inversion, is a non-equilibrium condition in a medium in which more particles occupy a higher-energy state than a lower-energy one, enabling net stimulated emission and light amplification. In thermal equilibrium the Boltzmann distribution prevents inversion; external pumping is required to sustain it.

In lasers, population inversion provides optical gain: the rate of stimulated emission exceeds the rate of absorption and nonradiative losses. Real lasers rely on multi-level schemes because two-level inversion is impractical. Four-level lasers are easier to sustain inversion because the lower laser level is rapidly depopulated to the ground state, reducing re-absorption. Three-level lasers, where the lower laser level is near the ground state, require higher pump power.

Creation of inversion occurs through pumping methods: optical pumping by intense light; electrical pumping in semiconductor lasers; chemical pumping in certain gas lasers. The pump rate, upper-state lifetime, and decay pathways determine whether inversion is possible at a given temperature and pressure.

Applications include numerous laser types: solid-state, gas, and semiconductor lasers; enabling technologies from communications to medicine. Limitations include losses, collisional dephasing, and reabsorption that reduce gain; maintaining inversion requires continuous energy input and careful design to minimize depopulation of the upper level or rapid decay of the lower level.

History: The concept arises from Einstein's A and B coefficients and was demonstrated experimentally with the development of laser action in 1960. Populaatioinversio remains a fundamental concept in quantum optics and laser physics.